Use of free silicon in liquid phase sintering of silicon nitrides and sialons

ABSTRACT

This invention relates to the production of improved high density nitrogen based ceramics by liquid-phase densification of silicon nitride or a compound of silicon-nitrogen-oxygen-metal, e.g. a sialon. In the process and compositions of the invention minor amounts of finely divided silicon are employed together with the conventional liquid phase producing additives to enhance the densification of the resultant ceramic.

The invention described herein was made in part under the followingFederal Grants: National Science Foundation DMR-79-24008 and Departmentof Energy DE-ACOZ-77ERO4386, creating certain rights in the UnitedStates Government.

This is a division of application Ser. No. 506,466, filed June 21, 1983,now U.S. Pat. No. 4,487,840.

BACKGROUND OF THE INVENTION

Liquid phase densification of nitrogen based ceramics is well known inthe art. The ceramic components are dry or wet mixed with liquid phaseforming oxide additives such as magnesia, alumina, yttria, and the like.The resultant powder compact is then densified by hot-pressing,hot-isostatic pressing, or pressureless sintering. The preparation ofnitrogen based ceramics by the above methods has been described, forexample, by Lange, International Metals Reviews, 1980 No. 1, pp. 1-19;Jack, J. Materials Science, 11(1976): 1135-1158; Terwilliger et al., J.Materials Science, 10(1975): 1169-1174; Mitomo, J. Materials Science,11(1976): 1103-1107; Greskovich, J. Am. Ceramic Soc., 64: 725-730; andSmith et al., Proc. of Symp. of Factors in Densification and Sinteringof Oxide and Non-oxide Ceramics, 1978, Japan.

The term liquid phase densification of ceramics is used to describethose processes where a small amount of a liquid phase present in theinter-particle interfaces of a powder compact leads to a significantenhancement in the densification rate as compared to the case where theliquid phase is not present. The mechanism of densification, in bothcases, is diffusional transport of atoms from particle-particleinterfaces to the interstitial pore regions formed by imperfect packingof particles in the powder compact. Densification is achieved as thepores are filled by the diffusion mechanism. The process is usuallycarried out at high temperatures. The liquid phase enhancesdensification by enhancing the diffusion rate of atoms along theinterfaces.

While the liquid phase densification of nitrogen-based ceramics hasshown promise, the attainment of maximum densities has been thwarted bya vaporization or volatilization phenomenon since apparently N₂ or otheroff gas is formed during the densification process, preventing maximumdensification. Thermodynamic calculations and experiments in very lowoxygen atmospheres have shown that one of the probable decompositionreaction is

    Si.sub.3 N.sub.4 (crystal)→3Si(liquid)+2N.sub.2 (gas)

The resultant gas bubbles in the liquid phase prevent completedensification of the powder aggregate.

It is noted that the process for preparing reaction-bonded-siliconnitride is not intended to be included with the scope of this invention.In that process, a compact of at least a predominant amount of siliconpowder is fired in a nitrogen atmosphere to produce silicon nitride. Theresultant product of this process generally is porous and has inferiormechanical strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b graphically present the results of Example 1.

FIG. 2 graphically presents the results of Example 2.

FIG. 3 graphically analyzes the influence of silicon activity andtemperature on the vapor pressure of nitrogen gas in a Si₃ N₄ system.

DESCRIPTION OF THE INVENTION

This invention relates to the production of improved high densitynitrogen based ceramics by liquid-phase densification of silicon nitrideor a compound of silicon-nitrogen-oxygen-metal, e.g. a sialon. In theprocess and compositions of the invention, minor amounts of finelydivided silicon, Si, are employed together with the conventional liquidphase producing additives to enhance the densification of the resultantceramic.

The powders which can be densified in accordance with this inventioninclude Si₃ N₄ powders as well as sialon powders. Sialons, for example,described by Jack, supra, are compounds derived from silicon nitridesand oxynitrides first prepared by simultaneous replacement of siliconand nitrogen by aluminum and oxygen. It is now known that other metalatoms can be incorporated and the term sialon has become a generic oneapplied to materials where the structural units are (Si,Al)(O,N)₄ or(Si,M)(O,N)₄ tetrahedra. These powders are generally employed in thedensifiable compositions in an amount which comprises between about 83%to about 99% and preferably about 93% to about 98% based on the totalcompositions. The powders employed are finely divided pulverulentmaterials typically having an average particle size of about 0.1 μm toabout 10.0 μm and preferably about 0.1 μm to about 1.0 μm.

It is noted that a key to the present invention is the addition ofsilicon (Si). It is further noted that at times silicon nitride powdersas produced have residual silicon in the central core region due toincomplete nitride formation. This residual silicon does not have thebeneficial effect on the densification process in the same manner as theuse of the free silicon powder as in the invention. The residual siliconin the core of the silicon nitride powder particles is isolated from theliquid phase, which wets the surface of the powder particles. Therefore,the use of silicon-nitride powder which contains residual silicon fallswithin the scope of this invention since the addition of free siliconwill still lead to an improvement in liquid phase densification.

It is further noted that nitrogen based ceramic materials such assilicon nitride frequently contain silica, SiO₂, as a surfacecontaminant; such materials are within the scope of the ceramic formingpulverulent materials employable in the compositions and method of theinvention.

In the process of the invention, known liquid phase producing bondingadjuvants are included in the compositions. These additives includemagnesia (MgO), yttria (Y₂ O₃); alumina (Al₂ O₃), aluminum nitride(AlN), and silica (SiO₂) or mixtures thereof. Generally, these additivesare employed in densification, enhancing amounts generally in the rangeof about 1% to about 7% by weight, based on the total composition. Theparticle size of these additives is usually generally between about 0.1μm and about 10.0 μm and preferably about 0.1 μm and about 1.0 μm.

The various components of the nitrogen based ceramic compositions,including the silicon can be mixed and blended employing the wet or drymixing techniques well known in the art.

The resultant mixture is then adapted and fired using conventionalceramic forming techniques and methods. Firing is conducted in anitrogen atmosphere at a temperature and a time sufficient to form ahigh density ceramic body generally having a density of at least about95% and preferably at least about 98% of theoretical density. Thetemperature employed is generally at least about 1600° C. and preferablyat least about 1700° C. The time varies with the components but routineexperimentation will readily determine the optimum time and temperatureto obtain maximum density.

Since as theorized hereinafter, the system is dynamic, the compositionspreferably should be brought to sintering temperature reasonably rapidlyand should not be held at that temperature substantially beyond the timerequired to achieve maximum density.

The compositions of the invention can be formed into ceramics byhot-pressing, hot isostatic pressing or pressureless sintering. However,since pressureless sintering has obvious advantages as to economics andthe size and shape of the resultant ceramics and since very high andeven theoretical densities can be obtained using the compositionsdescribed in the pressureless sintering process, that process ispreferred.

EXAMPLE 1

The following silicon nitride compositions were mixed and sintered:

    ______________________________________                                                       Composition                                                                   A       B       C                                              Material         Parts by weight                                              ______________________________________                                        Si.sub.3 N.sub.4 95        95      95                                         (99.9% purity - 325 mesh)                                                     MgO                5.sup.(1)                                                                               5.sup.(1)                                                                             5.sup.(2)                                Silicon           5        --      --                                         (99.5% purity - 325 mesh)                                                     mixing method    wet       wet     dry                                        ______________________________________                                         Notes:                                                                        .sup.(1) Added in the form of Mg(NO.sub.3).sub.2.6H.sub.2 O                   .sup.(2) Added as powder, 99.5% purity, 325 mesh.                        

The compositions were blended and mixed as follows:

Dry Mixing: The powders were weighed and added to isopropyl alcohol ofapproximately equal weight. The mixture was milled for 24 hours in aplastic bottle with tungsten-carbide balls. Dry powder was obtained byevaporating the isopropyl alcohol. The mixture was constantly stirredduring the drying process.

Wet-mixing: Magnesium nitrate was dissolved in isopropyl alcohol and theremaining powders were added to this solution. The mixture wasthoroughly blended and then dried on a hot plate with continuousstirring. The powder was crushed with a mortar and calcined at 700° C.for 51/2 hours. The calcined powder was carried through the dry-mixingprocedure described in the preceding paragraph.

The compositions were molded and fired as follows: The compositions werecold die-pressed into cylindrical specimens about 15 mm long and 12.5 mmin diameter. The specimens were sintered in nitrogen at one atmospherepressure. The final density was measured as a function of temperatureand time. The results are shown in FIGS. 1a and 1b.

In the Si₃ N₄ -5% MgO system 100% densification was achieved by theaddition of silicon whereas without silicon densities significantly lessthan 100% were measured under the same condition.

EXAMPLE 2

The following MgO doped sialon (Si₃ N₄ -SiO₂ -AlN-MgO) system ws mixedand sintered:

    ______________________________________                                                          Compositions                                                                  A     B                                                     Material            Parts by Weight                                           ______________________________________                                        Si.sub.3 N.sub.4    64      64                                                (99.9% purity - 325 mesh)                                                     Al.sub.2 O.sub.3    21      21                                                (99.8% purity - 0.25μ)                                                     AlN                 9       9                                                 (99% purity - size)                                                           SiO.sub.2           1       1                                                 (99.% purity - 325 mesh)                                                      MgO                 5       5                                                 (99.5% purity - 325 mesh)                                                     Si                  5       --                                                (99.5% purity - 325 mesh)                                                     ______________________________________                                    

The compositions were blended and mixed as follows:

The powder compositions were blended and mixed in isopropyl-alcohol ofapproximately equal weight. The mixture was milled for 24 hours in aplastic bottle with tungsten-carbide balls. Dry powder was obtained byevaporating isopropyl-alcohol. Continuous stirring was used during thedrying process.

The dry powder mixture was molded into cylindrical specimens 15 mm longand 12.5 mm in diameter, using a steel die for cold-pressing. Thespecimens were sintered in nitrogen at one atmosphere pressure for 30minutes. The final density was measured as a function of temperature.Using silicon, a hundred percent dense material was obtained in 30minutes at 1750° C. Without silicon, a maximum density of 87% wasobtained. The results are set forth in FIG. 2.

It is noted that the silicon which is added to enhance densification isconverted to silicon nitride during densification. This was shown bytransmission electron microscopy. It is theorized that the beneficialeffect of silicon in densification is obtained before the silicon isfully converted to silicon nitride. Densification in the presence ofelemental silicon in the liquid phase and nitridation of silicon appearto be competitive kinetic processes. The size of the silicon powderparticles, their weight fractions and uniformity of distribution willinfluence the relative kinetics. The rate of heating up and residencetime will also influence the maximum and final density of the sinteredproduct.

Thermodynamic analysis of reaction Si₃ N₄ (crystal)→3Si (liquid)+2N₂(gas) leads to the results shown in FIG. 3 which gives the vaporpressure of N₂ as a function of the activity of silicon and thetemperature. Note, for example, that increasing the activity of Si toone depresses the vapor pressure of nitrogen to 0.1 atmospheres at 1600°C. These calculations support the results obtained in that increasingthe activity of silicon appears to depress volatilization which in turnenhances final density.

We claim:
 1. In a process of forming a silicon nitride based ceramicthrough a liquid phase densification process wherein the ceramic formingcomposition contains at least about 83% silicon nitride having anaverage particle size ranging from 0.1 μm to about 10 μm and from about1% to about 7% by weight of the total composition of liquid phaseproducing oxide, other than aluminum oxide and wherein densificationinvolves sintering in nitrogen at a temperature ranging from about 1600°C. to about 1800° C. at one atmosphere, the improvement which comprisesincluding in said composition from about 1% to about 7% by weight of thetotal composition of pulverulent free silicon to enhance densificationto obtain at least about 98% of theoretical density in a sintering timeof up to about 30 minutes.
 2. A process as recited in claim 1 whereinthe liquid phase producing oxide is selected from the group consistingof magnesia, yttria, silica and mixtures thereof.
 3. A process asrecited in claim 2 wherein sintering is carried out at a temperature ofabout 1600° C. to about 1750° C.
 4. A process as recited in claim 2wherein sintering is carried out at a temperature of at least about1700° C.
 5. A process as recited in claim 2 wherein sintering is carriedout at temperature of at least about 1650° C.